As an antenna system related to a coaxial line slot array antenna, there is generally known a waveguide slot array antenna (for example, see Patent Document 1). In this waveguide slot array antenna, a waveguide, a short-circuit plate for short-circuiting both ends of the waveguide, and slots provided in a wide wall surface of the waveguide are combined to form a sub-array. There is provided a feed circuit as feeding means for the sub-arrays, and the respective sub-arrays and the feed circuit provided to the sub-arrays are combined to form a waveguide slot array type planar array antenna.
This antenna is uniformly excited when an input signal is uniformly transmitted to the feed circuit provided to the respective sub-arrays through a signal path. In a waveguide slot array which is a sub-array unit, the both ends of the waveguide are short-circuited by the short-circuit plate, and its length is set so that a standing wave propagates through the guide at a frequency to be used. The slots are set to have a length of substantially a half-wavelength, and are formed at desired intervals corresponding to standing wave excitation to be uniformly excited. Accordingly, the slots provided in the planar antenna are all uniformly excited, to thereby achieve high-gain radiation characteristics.
Further, there is provided means for performing phase control, and hence beam scanning can be performed. It should be noted that the reason why directions of the slots alternately differ is that the slots are formed at ½ λg (λg is a guide wavelength of the waveguide) interval on a tube axis. Further, depending on a polarized wave to be used, for example, the antenna may be used as one of a waveguide shunt slot array type (for example, see Patent Document 2).
It should be noted that, as a feature of the waveguide slot array antenna, in the case where the waveguide for exciting the slots is assumed to be a transmission line, first, loss is extremely lower compared with other line such as a microstrip line or a suspended line.
As an example of a coaxial line used for feeding, there is one in which one end of a probe is inserted into the coaxial line, and an element antenna is connected to another end thereof, to thereby perform feeding to the antenna (for example, see Patent Document 3). However, use of the probe complicates a structure and makes adjustment of a probe length difficult.
Patent Document 1: JP 62-210704 A
Patent Document 2: JP 2005-204344 A
Patent Document 3: JP 2000-209024 A